An electric motor is a machine that is used to convert electrical energy into mechanical energy. When electric current is passed through a current carrying conductor a magnetic field is produced in the area around it. The magnitude of current dictates the strength of the magnetic field that is produced. Magnetic fields are the fundamental mechanism by which energy is converted from one form to another in motors, generators and transformers.
Four basic principles describe how magnetic fields are used in these devices.                1) A current-carrying wire produces a magnetic field in the area around it.        2) A time-changing magnetic field induces a voltage in a coil of wire if it passes through that coil. (This is the basis of transformer action).        3) A current-carrying wire in the presence of a magnetic field has a force induced on it. (This is the basis of motor action).        4) A moving wire in the presence of a magnetic field has a voltage induced in it. (This is the basis of generator action).        
In the operation of an electric motor, electric power is delivered to a coil of wire (inductor) which is generally wound around an inductor core (ferromagnetic material and a path of integration used to focus the (primary) magnetic field on the motor's stator into a desired location such as another (secondary) magnetic field in order to effect a change in the kinetic energy of the second magnetic field) on the rotor.
Motor coils (inductors) can be made without cores; however the magnetic field produced by such motor coils lacks focus is divergent and weak and lacks the strength of a ferromagnetic cored inductor with its specifically focused magnetic field direction.
All electric motors currently alternate the motor coil's (primary) induced electromagnetic field polarity in concert with the rotor's (secondary) alternating magnetic field to affect motor action and produce mechanical power.
This (primary) induced electromagnetic field polarity alternation must be done very quickly and artificially because the motor coil's 5 time constant rise time would prohibit timely motor coil electromagnetic field pole orientation reversal.
As a result the electromagnetic field energy that is initially established by the motor coil; it must therefore be dissipated into free space before its magnetic field direction can be changed to produce an alternate electromagnetic field polarity as required by the rotor's alternating magnetic field.
The electromagnetic field alternation and free space energy dissipation occurs when the energy magnitude is at its maximum level and as a result the energy dissipation and negative effects are always at the maximum level possible.
Electromagnetic field radiation exposure as a result of the free space energy dissipation produces negative side effects when it comes in contact with living tissue and the exposure and negative side effects that result are cumulative over time.
Parasitic problems exist in all electric motors, including motor armature reaction/generator action in a motor which causes undesired effects such as neutral plane shift, flux weakening and the required solution interventions such as interpoles and compensating windings which add to the complexity and cost of electric motors but do not completely solve the problematic issues.
Armature Reaction in a motor is a problem that manifests itself when the magnetic field windings of a DC machine are connected to a power supply and the rotor of the machine is turned by an external mechanical power source. In this case a voltage will be induced in the conductors of the rotor and will be rectified into DC output by the action of the machine's commutator. When a load is connected to the terminals of the machine a current will flow in its armature windings and this current will produce a magnetic field of its own, which will distort the original magnetic field from the machine's poles. This distortion of the flux in a machine as the load increases is called Armature Reaction and it causes two serious problems in DC machines.
The first problem caused by Armature Reaction is Neutral Plane Shift where the magnetic flux in the air gap of the machine is skewed (variable); sometimes it subtracts from the pole flux and sometimes it adds to the pole flux. The amount of neutral plane shift depends on the amount of rotor current and hence the load on the machine. The problem associated with neutral plane shift manifests itself in the commutator when the commutator must short out commutator segments just at the moment when the voltage across these segments is equal to zero.
When the machine is loaded, neutral plane shift occurs and the brushes short out commutator segments when the voltage across them is not zero. The result is a current flow circulating between the shorted segments and large sparks at the brushes when the current path is interrupted as the brush leaves the commutator segments. The end result is arcing and sparking at the brushes which is a very serious problem since it leads to drastically reduced brush life, pitting of the commutator segments and greatly increasing maintenance costs.
Neutral plane shift cannot be fixed by placing the brushes over the full-load neutral plane, because then they would generate a spark at no-load. In extreme cases, the neutral plane shift can lead to Flashover in the commutator segments near the brushes. Flashover occurs when the air above brushes in a machine become ionized as a result of the sparking of the brushes.
Flashover occurs when the voltage of adjacent commutator segments gets large enough to sustain an arc in the ionized air above them. If Flashover occurs, the resulting arc can melt the commutator's surface. The second major problem caused by armature reaction is flux weakening.
Most machines operate at flux densities near the saturation point, therefore at locations on the pole surfaces where the rotor magnetomotive force adds to the pole magnetomotive force, only a small increase in flux occurs. But locations on the pole surfaces where the rotor magnetomotive force subtracts from the pole magnetomotive force, there is a large decrease in flux and the net result is that the total average flux under the entire pole face is decreased. Flux weakening causes problems in both generators and motors.
In generators the effect of flux weakening is simply to reduce the voltage supplied by the generator for any given load. A current carrying-conductor produces its own magnetic field, and this magnetic field affects the main magnetic field of the alternator. It has two undesirable effects, either it distorts the main field or it reduces the main field flux or both, which can deteriorate the performance of the machine. When the field gets distorted, it is known as cross magnetizing effect resulting in reduced flux known as demagnetizing effect.
The electromechanical energy conversion takes place through the magnetic field as a medium. Due to relative motion between armature conductors and the main field, an EMF is induced in the armature windings whose magnitude depends upon the relative speed as well as the magnetic flux. Due to generator armature reaction, flux is reduced or distorted, the net EMF induced is also affected and hence the performance of the machine degrades.
In motors, the effect can be more serious because when the flux in a motor decreases its speed increases and increasing the speed of a motor can increase its load, resulting in more flux weakening. It is possible for some DC motors to reach a runaway condition as a result of flux weakening, where the speed of the motor just keeps increasing until the machine is disconnected from the power line or until it destroys itself.
Problems associated with the current flow reversal and the timing required to accomplish this is called inductive kick. If we assume that a machine turning at 800 RPM having 50 commutator segments (a reasonable number for a typical motor), each commutator segment moves under a brush and must clear it again in 0.0015 seconds. With even a tiny inductance in the armature winding loop, a very significant voltage kick will be introduced in the shorted commutator segment and this high voltage naturally causes sparking at the brushes, resulting in the same arcing problems associated with neutral plane shift.
Two approaches are currently used to partially or completely correct the problems of armature reaction and inductive kick voltages; they are interpoles and compensating windings. The use of commutating poles or interpoles is very common because they correct sparking problems of DC machines at a fairly low cost however they do nothing for the flux distortion under the pole faces, so the flux weakening problem is still present. Most medium-size, general purpose motors correct for sparking problems with interpoles and just live with the flux-weakening effects. For very heavy, severe duty cycle motors, the flux-weakening problem can be a very serious issue.
In order to completely cancel armature reaction and eliminate both neutral-plane shift and flux weakening, a different technique was developed called compensating windings which requires the placement of compensating windings in slots carved into the faces of the poles parallel to the rotor conductors, to cancel the distortion effects of armature reaction. The major disadvantage of compensating windings is that they are very expensive, since they must be machined into the faces of the poles and because of the expense of having compensating windings and interpoles, these windings are used only where the extremely severe nature of a motor's duty demands them.